The present invention relates to a selective metal electrodeposition process and apparatus.
Following is a list of the most important requirements for metallization processes for advanced semiconductor processing:
low metal resistivity (comparable to or less than that of aluminum)
low contact resistance to the active device area (a requirement of the diffusion barrier and not the current carrying metal)
low contact resistance to previous and subsequent metallization steps
excellent contact and via step coverage capability
acceptable film morphology, adhesion and stress properties
decreased line-to-line capacitance
improved planarization processing
compatibility with planarization processing
compatibility with other processing (such as salicides and dielectric depositions)
competitive economics and throughput considerations
excellent reliability (electromigration and corrosion concerns)
environmentally responsible process
Sputtered aluminum has electromigration and step coverage concerns. Electromigration is an atomic transport mechanism which allows metal atoms to move due to an applied direct current resulting in the formation of voids in a metal line. These voids can cause an increase in line resistance and ultimately the opening of a line (open circuit). Step coverage describes the ability of the metal to fill contact and via holes. This directly affects the ability of the metal to carry current into and out of the contacts and vias. Poor step coverage may lead to the failure (open circuit) of the metal in the hole.
Subsequent processing (planarization and stacking vias on top of contacts) is also complicated by poor step coverage. Layering the aluminum with a more electromigration resistant metal or alloying the aluminum (forming new phases at the sensitive grain boundaries) are two approaches to reduce the probability of this failure mechanism. They are only partial solutions and introduce other problems such as complicating the metal etch step and increasing the sheet resistance of the metal. Solving the step coverage problem with standard sputtering techniques has also convincingly failed.
LPCVD (low pressure chemical vapor deposition) aluminum addresses the step coverage issue (LPCVD can provide excellent step coverage) but raises concerns about electromigration resistance, film morphology (optimum grain size for electromigration resistance) and corrosion resistance. The ability to alloy the aluminum using this technique has not yet been demonstrated nor has its production worthiness.
Hot sputtered aluminum is of questionable production worthiness and electromigration resistance.
Reflowed aluminum requires processing with an expensive laser ($700,000) after standard sputtering. Excellent step coverage is promised, but all problems relating to electromigration are still present.
Blanket LPCVD tungsten is presently economically unattractive as well as exhibiting a much larger resistivity than that of aluminum. Some economic improvements can be expected, but its large resistance requires thick films which results in large fringing (line to line) capacitances which degrade device performance.
Thick lines and lines with sharp edges and corners also complicate subsequent planarization processing. Only a small fraction of current and future process flows will be able to cope with the performance limitations and processing complexities of this metallization scheme. Additionally, plasma etching of tungsten has not proven to be as production worthy a process as that of aluminum etching.
Selective LPCVD tungsten plugs with sputtered aluminum addresses the step coverage problem, but has throughput and economic problems, leakage current concerns when used as the first metallization process and electromigration resistance concerns. The relatively new silane reduction process should address some of the throughput and economic concerns and possibly the Junction leakage concerns. Step coverage should be excellent, but electromigration resistance is still an issue.
The selective metal electrodeposition process (utilizing copper) yields a metal with excellent resistivity, approximately 2 micro-ohm-centimeters (this value is superior to that of aluminum, approximately 3 micro-ohm-centimeters).
Step coverage can be manipulated with the deposition parameters (similar to an LPCVD process. The added benefit of adding leveling agents to the electrolyte further aids in achieving excellent step coverage. The economics of an electrochemical deposition process must be compared to other metallization processes. No vacuum pumps, mass flow controllers, sophisticated high temperature furnaces and controllers, toxic gases or toxic gas detectors are necessary.
These facts lead one to conclude that an economical piece of equipment can be manufactured once a production worthy process is established. The simplification of the metal etch step would also have favorable economic consequences. Experimental deposition rates have also been favorable with respect to throughput requirements. The reliability of a copper metallization process must be proven. The fact that copper is already used as an interconnect metal in semiconductor packaging is a beneficial fact. It is also one of the most common and studied metals.
Concerns over other failure mechanisms at the smaller geometrics associated with this process will have to be examined. Certainly the failure mechanisms relating to atomic transport mechanisms (electromigration) are of minimal probability due to the relatively high melting point of copper (compared to that of aluminum).
It is an object of the present invention to provide an improved electrodeposition process.
A selective metal electrodeposition process and its associated hardware will be described which solves the problems associated with current and proposed metallization schemes. The advantage of this process alluded to in Table I will be discussed in detail.
Briefly, in one preferred embodiment, the present invention is directed toward a process for electrodeposition of metal such as copper, silver or gold into a semiconductor wafer having an active layer such as silicon. The process completes the steps of a diffusion barrier deposition process to provide an electrically conducting layer having contacts and vias on said wafer and to prevent metal diffusion to said silicon, a nucleation layer process to provide a sufficient adhesion surface for the electrodeposited metal, an inert metal mask process to place a first metal mask layer onto said wafer, a selective electrodeposition process to selectively electrodeposit said metal onto said conducting layer of said semiconductor wafer while simultaneously completely filling a contact or via without a standard resist metal etch, a photoresist removal step, and an electrochemical etch process to decrease line-to-line capacitance and simplify planarization processing.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description which follows and in part become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the present invention may be realized and attained by means of the instrumentalities and combinations which are pointed out in the appended claims.